1. Field of the Invention
The present invention relates generally to an image forming system such as a copying machine, a printer or the like, and more particular to apparatus for feeding a flexible, sheet-like transfer medium, and which is incorporated in an image forming system adapted to expose the transfer medium to an electric charge.
2. Description of the Prior Art
It has heretofore been well known that an image forming system is usually arranged in such a manner that a flexible, sheet-like transfer medium is brought into contact with or in close proximity to the toner image which is electrostatically formed on an image carrier such as an electrophotographic light-sensitive member, and that the transfer medium is exposed to an electric charge of a polarity opposite to that of the toner of the toner image, such exposure being effected by a transfer charger disposed close to the outer periphery of the light-sensitive member, so that the toner image is transferred to the transfer medium.
In general, such an image forming system is constructed so that a plurality of metal plates are disposed as guide plates having a suitable gap therebetween in the vicinity of the transfer charger and the transfer medium is fed, through the gap between the guide plates, to a transfer section which includes the transfer charger. Therefore, while the transfer medium is being fed, although the leading end of the medium reaches the position of the transfer charger, the trailing end portion of the medium occasionally remains within the gap in contact with the guide plates which are made of electrically-conductive metal.
In this state, since the transfer medium is exposed to an electric charge by the transfer charger as described above, when humidity is high, the level of resistance of the transfer medium tends to decrease, and thus electric charge leaks from the transfer charger to the guide plates through the transfer medium. If the guide plates are grounded, transfer voids are often formed in the toner image on the transfer medium by the leakage of the electric charge. If the guide plates are allowed to float electrically with respect to the other components, electric charge is accumulated within the guide plates causing floating toner to cling to the plates. Accordingly, there is a risk of the subsequent transfer medium being stained with such toner or the image forming system being caused to malfunction by the noise generated when the guide plates discharge the accumulated electric charge.
On the other hand, when humidity is low, if the guide plates are allowed to float electrically, the electric charge accumulated by the physical friction between the transfer medium and the guide plates will occasionally give rise to stains on the transfer medium or to system malfunction.
In order to eliminate such disadvantages, the following methods have heretofore been proposed. In one typical method, a suitable level of bias voltage is applied to the guide plates which are allowed to float electrically by known means. In another typical method, the guide plates are grounded via a voltage-regulation device or a resistor. However, these respective prior-art methods need an additional mechanism, together with associated wiring and safety measures for the protection of operators as occasion demands. Thus, neither of these prior-art proposals can be considered as a satisfactory solution when factors such as space and cost are taken into account.